Searching for a long time and finally found the following:
/**
* [1] Reset the font-size to the root/default size.
* Note: Won't work in IE8 and below
* To make it work in IE8 and below just change to "font-size: $my-font-size;".
*/
.grid__item > * {
font-size: 1rem; /* [1] */
}
/**
* [1] Font-size 0 removes the white spaces. Won't work on mobiles..
* ..because the minimum font-size is 8px.
*/
.grid__item, .grid {
font-size: 0; /* [1] */
}